The chemical adsorption isotherm reveals information about the active surface of a material and has been employed for many years as a standard analytical tool for the evaluation of catalysts. Temperature-programmed reaction techniques have emerged from the 1950’s as an indispensable companion to chemisorption isotherm analyses in many areas of industry and research. This paper provides an introduction to these analytical techniques.
Optimum design and efficient utilization of catalysts require a thorough understanding of the surface structure and surface chemistry of the catalytic material. Chemical adsorption (chemisorption) analyses can provide much of the information needed to evaluate catalyst materials in the design and production phases, as well as after a period of use. The required analytical equipment can be relatively inexpensive, simple to operate, and fast compared to alternate equipment capable of obtaining the same information.
A solid material usually exhibits a heterogeneous distribution of surface energy. Gas, vapor, or liquid molecules may become bound to the surface if they approach sufficiently close to interact. The discussions in this paper are confined to the adsorption (and desorption) of gases or vapors on (or from) solid surfaces. The solid is called the adsorbent; the gas or vapor molecule prior to being adsorbed is called the adsorptive and while bound to the solid surface, the adsorbate.
Physical adsorption is the result of a relatively weak solid-gas interaction. It is a physical attraction resulting from nonspecific, relatively weak Van der Waal’s forces and adsorption energy usually not exceeding 80 kJ/mole, with typical energies being considerably less. Physically adsorbed molecules may diffuse along the surface of the adsorbent and typically are not bound to a specific location on the surface. Being only weakly bound, physical adsorption is easily reversed.
Adsorption also can result in a surface complex, a union much stronger than a physical bond with heats of adsorption up to about 600 kJ/mole for C-N bonds and 800 kJ/mole for chemical bonds. A chemical bond involves sharing of electrons between the adsorbate and the adsorbent and may be regarded as the formation of a surface compound. Due to the bond strength, chemical adsorption is difficult to reverse.
Physical adsorption takes place on all surfaces provided that temperature and pressure conditions are favorable. Chemisorption, however, is highly selective and occurs only between certain adsorptive and adsorbent species and only if the chemically active surface is cleaned of previously adsorbed molecules.
Under proper conditions, physical adsorption can result in adsorbed molecules forming multiple layers. Chemisorption, in the typical case, only proceeds as long as the adsorptive can make direct contact with the surface; it is therefore a single-layer process. Exceptions can exist if the adsorptive is highly polar such, NH3 being an example. Both physical and chemical adsorption may occur on the surface at the same time; a layer of molecules may be physically adsorbed on top of an underlying chemisorbed layer. The same surface can display physisorption at one temperature and chemisorption at a higher temperature. For example, at liquid nitrogen temperature (77 K) nitrogen gas is adsorbed physically on iron but at 800 K, an energy level too high for physical adsorption bonds, nitrogen is adsorbed chemically to form iron nitride (Moore).
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